M.C.limbachiyaH. Y.kew

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Physical Properties of Low Density Aircrete Products

Mukesh C. Limbachiya1, Hsein-Yang Kew

2

1 Professor; Head of School; Director of Engineering Research2 Senior Lecturer

School of Civil Engineering & Construction, Kingston University, London, UK

Abstract: Currently in the UK, aircrete, also known as Autoclaved Aerated Concrete, masonryunits are usually produced with a minimum declared compressive strength of 2.9 N/mm2

(measured to EN 772-1) and a minimum density of 400 kg/m 3. However, newly introduced

European Standards allow the use of lower strength and with this it is possible to produce lowerdensity products, such as low-density aircrete with a minimum declared compressive strength of2.0 N/mm2and a density of 350 kg/m3. This paper provides details of an extensive investigation

to assess key physical properties such as moisture properties, freeze/thaw resistance, thermalperformance, combustibility and flanking sound performance of low-density aicrete products. The

results show that key physical properties of low-density aircrete were comparable to conventionalaircrete products.

Keywords: physical properties, Low Density Aircrete

1. INTRODUCTION

Aircrete was first developed in Sweden in 1924 and first used in the UK in late

1950s as an alternative to building with timber (H+H, 2003). Currently, Aircrete is

being used extensively by major house builders in the UK that Aircrete (mainly medium

and high density) now accounts for a third of all concrete blocks with an approximately30 million m2of Aircrete block sales annually (Construction Markets, 2000).

Aircrete is produced by mixing cementitious materials, cement and/or pulverised

fuel ash (PFA), lime, sand, water and aluminium powder. The final process involves

autoclaving for approximately 10 hours at high temperature and pressure (Mitsuda and

Kiribayashi, 1992; Isu and Mitsuda, 1992 and Pospisil, 1992), hence, the material is

also known as AAC (Autoclaved Aerated Concrete) commercially. The solid material

part, which consists of the crystalline binder called Tobermorite by mineralogist,

constituent bulk of the binder with grains of Quartz and some other minerals are found

in minor amounts (Aircrete Bureau, 2003). It is the Tobermorite, which comprises of

silicium dioxide, calcium oxide and water that provides the high compressive strength

of Aircrete in spite consisting high proportion of pores, which is about 60 85% of airby volume. Therefore, opportunity exists to use the relatively higher porosity of Low

Density Aircrete, which consists of 70-85% of air by volume, produced with sufficientstrength and lower density (typically of 350 kg/m3) for dwellings construction.

The lightweight porous structure and easily manhandled of Aircrete blocks,

provides significant improvement in productivity, thus enhancing the efficiency of

construction and cost effective, in line with Egans report on Rethinking Construction(Egan, 1998). Key physical properties of Low Density Aircrete with a declared

compressive strength of 2.0 N/mm2(2.0N) and a density of 350 kg/m

3were investigated

and compared with the conventional Aircrete blocks with a declared compressive

strength of 2.9 N/mm2(2.9N) and a density of 460 kg/m

3.


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2. TEST MATERIALS AND EXPERIMENTAL PROGRAMME

Two types of Aircrete blocks, namely (a) Low Density Aircrete of 620 x 150 x 250

mm with a compressive strength of 2.0 N/mm2(2.0N) and a density of 350 kg/m

3and

(b) Aircrete of 440 x 150 x 215 mm with a compressive strength of 2.9 N/mm2(2.9N)

and a density of 475 kg/m3were used. A series of experimental work was carried out in

accordance with appropriate British Standard.

Table 1. Key physical properties experimental programme

Physical properties Testing

Dimension BS EN 772: Part 16: 2000

Density BS EN 772: Part 13: 2000

Moisture content by mass BS EN 772: Part 10:1999Water absorption BS EN 772: part 11: 2000

Water absorption transmission BS EN ISO 12572: 2001

Moisture movement BS EN 680: 1994

Freeze/thaw resistance British Board of Agrment MOAT 12: 1977

Thermal performanceBS EN 12664: 2001 (verified by reference toBS EN 1745: 2002)

Combustibility BS EN ISO 1182:2002 and BS EN ISO 1716: 2002

Sound performanceThis was carried out at Acoustic Test Laboratory inaccordance with Part E of Building Regulations

3. TEST RESULTS AND DISCUSSION

3.1. Dimensions

The average measured dimensions for both 2.0N and 2.9N Aircrete masonry units

were impressively within 3.0mm of the theoretical values, as shown in Table 2.

3.2. Density

The average measured density for 2.0N and 2.9N Aircrete masonry units was 350kg/m

3and 475 kg/m

3respectively, which is remarkably within 0.57% and 0.42% of the

desired values, as summarised in Table 2.

Table 2. Average dimensions and density of 2.0N and 2.9N

AircreteAverage values

length (mm) width (mm) height (mm) density (kg/m3)

2.0 619.8 149.8 249.8 3522.9 439.7 149.8 214.8 477

3.3. Moisture properties

Four different test methods have been used to assess and examine the moisture

properties of 2.0N and 2.9N Aircrete units. These include moisture content by mass,

water absorption properties, water vapour transmission properties and moisture

movement. The test results are summarised in Table 3.


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The results show that the moisture contents of both types of Aircrete blocks arewithin the standard requirement of 6-30%. The coefficient of water absorption, Cw,s was

found to be greater for 2.9N Aircrete. This is as expected as the rate of absorption is

determined by the dry density of the material, greater Cw,s for the higher density

materials (AAC, 1978 and Zhou et. al., 1992). Likewise, water vapour flow rate (G) anddensity of water vapour flow rate (g) was found to be greater for 2.9N Aircrete. The

results suggest that the higher density material has a greater water vapour flow rate,

hence, a higher density of water vapour flow rate. The results also show that higher

density material has a greater drying shrinkage rate, as shown in Figure 1. However, the

drying shrinkage rates of both Aircrete blocks are relatively very low, which equates to

approximately 1 mm expansion for a 6 m wall.

3.4. Freeze/thaw resistance

The compressive strength and loss of volume of 2.0N Aircrete after 0, 20, 40 and

60 of freeze/thaw cycles are summarised in Table 4. The data show that the meanvolume change (loss) was > 5% of the original samples volume after 60 cycles. The

result exceeded the specified maximum limit. The majority of the volume loss occurred

to the blocks load bearing surface that was immersed in the 25mm water in the trays.

Therefore, the compressive strength has been calculated (1) using the area of load

bearing surface remaining of n number of freeze/thaw cycles and (2) using the original

load bearing surface are prior to testing. A small increase was measured for the former(1) but a 30% decrease for the latter (2) and hence, exceeded the maximum specified

loss in strength. The poor performance of 2.0N Aircrete subjected to freezing and

thawing cycles could be attributed to the high volume of pores.

Table 3. Moisture properties test results

Aircrete

blocks(N/mm2)

Av. moisture

content(% by mass)

Coefficient of waterabsorption after long time

exposure

Water absorptiontransmission

Av. drying

shrinkage

(x 10-6)Time

(s)

Cw,sg/(m2x s0.5)

G(kg/s)

(x 10-8)

G

(kg/m2s)

(x 10-7)

2.0 13.70

616800 32.5

2.24 2.89 300

673320 28.1

3293220 21.6

522300 27.9

1274860 28.0

1383060 20.4

2.9 13.60

699960 53.2

2.52 5.34 455

1033200 41.81440900 36.3

1903800 32.0

1199460 37.9

2684220 41.2


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-500

-450

-400

-350

-300

-250

-200

-150

-100

-50

0

0 20 40 60 80 100 120

Strain

(10-6)

Moisture content (%)

2.9N 2.0N

Sample 1 Sample 1

Sample 2 Sample 2

Sample 3 Sample 3

Sample 4 Sample 4

Sample 5 Sample 5

Fig. 1. Moisture movement for 2.0N and 2.9N Aircrete blocks

Table 4. Compressive strength and loss of volume of 2.0N Aircrete blocks after freeze/thaw cycles

Freeze/thawcycles

Mean compressive

strength(1)ofremaining Aircrete

(N/mm2)

Equivalent meancompressive strength(2)

of Aircrete (N/mm2)

Change of strength

(%)Loss ofvolume

(%)(1) (2)

0 2.20 2.20 N/A N/A N/A

20 2.30 2.09 + 4.5 - 6 1.5

40 2.20 1.74 + 0.0 - 21 5.1

60 2.25 1.54 + 2.3 - 30 6.4

3.5. Thermal performance

The test results in determining the thermal conductivity () of Aircrete blocks

show that 2.0N Aircrete blocks has a lower value of 0.10 W/mK as compared to 2.9NAircrete blocks, which has a value of 0.11 W/mK. The result is expected considering

that the lower density of 2.0N Aircrete blocks has a higher proportion of pores than that

of 2.9N Aircrete blocks. This thermally efficient of 2.0N Aircrete blocks offers greatopportunities in providing an improved thermal insulation of dwelling; hence helping to

meet the stringent thermal requirement of the UK Building Regulations Part L1 and J.

3.6. Combustibility

Table 5 and 6 summarises the fire resistance and gross calorific potential test

results, respectively. The behaviour of the samples during the tests was observed and no

difficulties were experienced. The 2.0N Aircrete blocks, in relation to its reaction to fire


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performance requirements relating to energy conservation and creating greenerconstruction in assisting the UK Government to tackle global warming. The application

of Aircrete blocks as modern construction material will significantly alleviated the

current problems in the UK construction industry, which is facing growing skills

shortages with the repercussions of rising costs that can affect build quality,sustainability issues, completion times and costs and may even impact on health and

safety.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the funding for the project provided by theEngineering and Physical Sciences Research Council (EPSRC) and the Industrial

Collaborators, Autoclaved Aerated Concrete Product Association, H+H Celcon,

Hanson-Thermalite, Quinn-Group, Tarmac Topblock, Building Research Establish-ment, Catnic-Corus UK, National House-Building Council and the Office of the Deputy

Prime Minister.

BIBLIOGRAPHY

[1] Aircrete Bureau, 2003. Code of Best Practice for the Use of Aircrete Products.

[2] AAC, 1978. CEB Manual of Design and Technology, The Construction Press,

Lancaster, London, New York.

[3] British Standard Institution, BS EN 772: Part 1: 2000, Methods of Test for

Masonry Unit Part 1: Determination of Compressive Strength, London.

[4] British Standard Institution, BS EN 772: Part 11: 2000, Methods of Test for

Masonry Unit Part 11: Determination of Water Absorption of Aggregate

Concrete, Manufactured Stone and Natural Stone Masonry Units due to CapillaryAction and the Initial Rate of Water Absorption of Clay Masonry Units, London.

[5] British Standard Institution, BS EN 772: Part 10: 1999, Method of Test for

Masonry Unit Part 10: Determination of Moisture Content of Calcium Silicate

and Autoclaved Aerated Concrete Units, London.


Masonry Unit Part 13: Determination of Net and Gross Dry Density of MasonryUnits (Except for Natural Stone), London.


Masonry Unit Part 16: Determination of Dimensions, London.

[8] British Standard Institution, BS EN ISO 12572: 2001, Hygrothermal Performance

of Building Materials and Products Determination of Water VapourTransmission Properties, London.

[9] British Standard Institution, BS EN 680: 1994, Determination of the DryingShrinkage of Autoclaved Aerated Concrete, London.

[10] British Board of Agrement, MOAT 12: 1977, The Assessment of Precast

Insulating Concrete Blocks for General Use in Building Assessment of

Freeze/thaw Resistance.[11] British Standard Institution, BS EN 1745: 2002, Masonry and Masonry Products

Methods for Determining Design Thermal Values, London.


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[12] British Standard Institution, BS EN 12664: 2001, Thermal Performance ofBuildings Determination of Thermal Resistance by Hot-Box Method Using Heat

Flow Meter Masonry, London.

[13] British Standard Institution, BS EN ISO 1182: 2002, Reaction to Fire Tests for

Building Products Non-combustibility, London.[14] British Standard Institution, BS EN ISO 1716: 2002, Reaction to Fire Tests for

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[15] British Standard Institution, BS EN 13501: Part 1: 2002. Fire Classification of

Construction Products and Building Elements: Classification Using Test Data

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[16] Construction Markets, 2000. The Market for Concrete Blocks in UK.[17] Egan J., 1998. Rethinking Construction, Report of the Construction Task force on

the Scope for Improving Quality and Efficiency in UK Construction. London.

[18] H + H Celcon Ltd., 2003. Celcon House, Ightham, Sevenoaks, Kent, TN15 9HZ.Available on www.celcon.co.uk/downloads/RIBA%20CPD.pdf.

[19] Isu N., Mitsuda T., 1992. Influence of Quartz Particles Size on the Chemical and

Mechanical Properties of Autoclaved Lightweight Concrete, Advances

in Autoclaved Aerated Concrete, Wittmann (ed.), Balkema, Rotterdam. ISBN 90

5410 086 9.

[20] Mitsuda T., Kiribayashi T., 1992. Influence of Hydrothermal Processing on the

Properties of Autoclaved Aerated Concrete, Advances in Autoclaved Aerated

Concrete, Wittmann (ed.), Balkema, Rotterdam. ISBN 90 5410 086 9.

[21] Pospisil F., 1992. Unit Weight Reduction of Fly Ash Aerated Concrete, Advances

in Autoclaved Aerated Concrete, Wittmann (ed.), Balkema, Rotterdam. ISBN 90

5410 086 9.

[22] The Building Regulations, 2000. Approved Document Part L1: Conservation of

Fuel and Power 2010 ed., The Stationery Office, London.[23] The Building Regulations, 2000. Approved Document Part J: Conservation of Fuel

and Power 2010 ed., The Stationery Office, London.

[24] The Building Regulations, 2000. Approved Document E: Resistance to the

Passage of Sound 2003 ed., The Stationery Office, London.

[25] Zhou W., Feng N., Yan G., 1992. Fracture Energy Experiments of AAC and its

Fractal Analysis, Advances in Autoclaved Aerated Concrete, Wittmann (ed.),Balkema, Rotterdam. ISBN 90 5410 086 9.

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M.C.limbachiyaH. Y.kew